The deal was negotiated between James Spring of Ealing Studios and Paul Kingsley of Lionhead Productions.
The film will star Maggie Smith, Hugh Bonneville, Timothy Spall and Anne Reid. The screenplay, also written by Fellowes, is based Lucy M Boston's The Children Of Green Knowe books, and tells the story of a boy who journeys through time to uncover family secrets.
From Time To Time will shoot on location in and around London and at Althelhampton House in Dorset.
Fellowes, Liz Trubridge and Paul Kingsley will produce.
After a career as an Oscar-winning screenwriter on projects such as Gosford Park, Fellowes made his directorial debut in 2005 with Separate Lies.
From Time To Time becomes the third film on Ealing Studios International's sales slate after I Want Candy and St Trinian's. The sales company will have an office in Cannes for the first time, led by Ealing's head of sales Natalie Brenner.
Ealing Studios Managing Director James Spring said: 'We're very pleased to be working with Julian and such a talented creative team on this project and are really excited to add this film to our Cannes slate.'